Zinc and zinc-alloy hot-dip-coated steel sheets are mainly used for automobile bodies because of low cost and excellent corrosion resistance, and in addition to the corrosion resistance due to coating, coating adhesion during press working is required for applying the steel sheets to automobile bodies. When coating adhesion deteriorates, coated layers peel as a powder or blocks, which phenomenon sometimes causes galling in press forming or deteriorates corrosion resistance of the portions from which the coated layer peels; and also, peeled fragments disadvantageously inflict the steel sheet.
As a conventional technique for improving coating adhesion, Japanese Patent Laid-Open No. 61-276961 discloses a technique in which alloying Fe with Zn at a high temperature ranging from 700 to 850.degree. C. is performed after zinc hot-dip-coating. However, alloying at a high temperature lead to not only higher costs but also increased expenses for equipment such as rolls.
Additionally, in Japanese Patent Laid-Open No. 3-232926, steel contains at least one of Zr, La, Ce, Y, and Ca, and the cooling rate from recrystallization annealing to coating is set to not less than 50.degree. C./sec. The cost is raised due to the addition of Zr or the like to steel and productivity deteriorates because the sheet-feeding rate has to be lowered due to the cooling capacity.
Furthermore, in Japanese Patent Laid-Open No. 2-163356, the O, Al, and N contents in steel are set to not more than 0.0045 wt %, (25.times.N wt %) to 0.15 wt %, and not more than 0.0030%, respectively. Moreover, restrictions on the Ti, Si, and P contents, and Si (wt %)+P (wt %).gtoreq.Ti (wt %) must be satisfied according to Japanese Patent Laid-Open No. 6-81101. Anyway, the desired steel-sheet properties such as strength and drawing cannot be always achieved by such content restrictions, and there is a possibility that coating adhesion will deteriorate because of deviations from a predetermined composition range.
In Japanese Patent Laid-Open No. 4-333552, coating adhesion is improved by carrying out Ni pre-plating before galvanizing. However, in general, a continuous galvanizing line (hereinafter referred to as "CGL") does not have such equipment, and a large investment is required for improving equipment or the like.
Meanwhile, automobile bodies are required to be lighter because of recent regulations for exhaust gas. Thinning the steel sheets is a method for lightening the automobile bodies. According to this method, it is necessary for ensuring safety to increase steel-sheet strength corresponding to the decreased thickness. Thus, high tensile-strength steel sheets have been developed for strengthening the steel sheets by increasing the steel contents of elements such as Si, Mn, and P. Since steel sheets for automobiles are subjected to press forming, excellent material characteristics with a high r-value (high Lankford value) are required, and in particular, the addition of these elements is essential for high-tensile strength steel sheets.
In the case of zinc hot-dip-coating such steel sheets, recrystallization annealing at a high temperature ranging from approximately 700 to 900.degree. C. is necessary to attain excellent material characteristics. In the CGL, recrystallization annealing is generally carried out under a nitrogen atmosphere in the presence of hydrogen (hereinafter referred to as reduction annealing), and although this atmosphere is a reducing atmosphere for Fe, it is an oxidizing atmosphere for some elements such as Si, Mn, and P. Thus, elements such as Si, Mn, and P (referred to as readily oxidizable elements) which are more oxidizable than Fe externally diffuse during reduction annealing and bond to oxygen on the surface of steel sheets to form oxides (called as "surface segregated layer"). Since these oxides significantly impede wettability between molten zinc and the steel sheets, so-called bare spots, i. e., defects occurring when zinc does not adhere to the steel sheets, are seen.
For overcoming such problems, Japanese Patent Examined Publication No. 61-9386 proposes a method of pre-plating the surface of steel sheets with Ni before the zinc hot-dip-coating process. However, according to this method, when steel contains at least Si and one more element among 0.2 to 2.0 wt % of Si, 0.5 to 2.0 wt % of Mn, and 0.1 to 20 wt % of Cr, Ni plating of not less than 10 g/m.sup.2 is necessary, resulting in an increased cost. In addition, although such a large quantity of Ni plating improves the wettability between the zinc hot-dip-coating and the steel sheet, disadvantageously, defects caused by Si and Ni on the coated surface frequently appear during the alloying process.
Furthermore, for example, Japanese Patent Laid-Open No. 57-70268 proposes a method of pre-plating the surface of steel sheets with Fe before the zinc hot-dip-coating process. According to this method, bare spots in Si-containing steel are preventable by p-e-plating, however, not less than 5 g/m.sup.2 of Fe plating is required, which fact is extremely uneconomical.
Additionally, other methods are disclosed in Japanese Patent Laid-Open Nos. 55-122865 and 4-254531. In these methods, steel sheets are oxidized beforehand to form a Fe oxide film on their surface and then subjected to reduction annealing. However, according to these methods, alloy elements such as Si are segregated on the surface to form an oxide film because of excess reducing during reduction annealing, causing a problem of inferior coating. For preventing such excess reducing, a large amount of Fe oxide is necessary. However, if the amount of Fe oxide film is exceedingly large, the Fe oxide film peels due to rolling or the like, thus on the contrary, a surface segregated layer is produced and results in impeded coating or adverse effects on operation because the fragments of the peeled Fe oxide-film are scattered inside a furnace.
In addition, concerning known proposals for the steel composition and hot-rolling conditions for zinc hot-dip-coating the high-tensile steel sheets, Japanese Patent Laid-Open No. 6-158172 discloses a method in which a steel containing Si.ltoreq.0.2 and Mn.ltoreq.1.5 by wt % is wound at a temperature not less than 650.degree. C. followed by acid washing, cold-rolling, annealing, and zinc hot-dip-coating; and Japanese Patent Laid-Open No. 6-179943 discloses a method in which a steel containing 0.10 to 1.5 wt % of Si and 1.00 to 3.5 wt % of Mn is wound at a temperature ranging from 500.degree. C. to 680.degree. C., both inclusive, followed by acid washing, cold-rolling, annealing, and zinc hot-dip-coating.
Although these methods give specified processing conditions, such as the steel composition and hot-rolling conditions, for a series of manufacturing steps, they cannot suppress the surface segregated layer formed during reduction annealing or improve bare spots or coating adhesion.